1. Field of the Invention
The present invention relates to wireless communications, and more particularly, to a method and apparatus for allocating a ranging channel for a synchronized mobile station in a wireless communication system.
2. Related Art
The institute of electrical and electronics engineers (IEEE) 802.16e standard was adopted in 2007 as a sixth standard for international mobile telecommunication (IMT)-2000 in the name of ‘WMAN-OFDMA TDD’ by the ITU-radio communication sector (ITU-R) which is one of sectors of the international telecommunication union (ITU). An IMT-advanced system has been prepared by the ITU-R as a next generation (i.e., 4th generation) mobile communication standard following the IMT-2000. It was determined by the IEEE 802.16 working group (WG) to conduct the 802.16m project for the purpose of creating an amendment standard of the existing IEEE 802.16e as a standard for the IMT-advanced system. As can be seen in the purpose above, the 802.16m standard has two aspects, that is, continuity from the past (i.e., the amendment of the existing 802.16e standard) and continuity to the future (i.e., the standard for the next generation IMT-advanced system). Therefore, the 802.16m standard needs to satisfy all requirements for the IMT-advanced system while maintaining compatibility with a mobile WiMAX system conforming to the 802.16e standard.
Effective transmission/reception methods and utilizations have been proposed for a broadband wireless communication system to maximize efficiency of radio resources. An orthogonal frequency division multiplexing (OFDM) system capable of reducing inter-symbol interference (ISI) with a low complexity is taken into consideration as one of next generation wireless communication systems. In the OFDM, a serially input data symbol is converted into N parallel data symbols, and is then transmitted by being carried on each of separated N subcarriers. The subcarriers maintain orthogonality in a frequency dimension. Each orthogonal channel experiences mutually independent frequency selective fading, and an interval of a transmitted symbol is increased, thereby minimizing inter-symbol interference.
When a system uses the OFDM as a modulation scheme, orthogonal frequency division multiple access (OFDMA) is a multiple access scheme in which multiple access is achieved by independently providing some of available subcarriers to a plurality of users. In the OFDMA, frequency resources (i.e., subcarriers) are provided to the respective users, and the respective frequency resources do not overlap with one another in general since they are independently provided to the plurality of users. Consequently, the frequency resources are allocated to the respective users in a mutually exclusive manner. In an OFDMA system, frequency diversity for multiple users can be obtained by using frequency selective scheduling, and subcarriers can be allocated variously according to a permutation rule for the subcarriers. In addition, a spatial multiplexing scheme using multiple antennas can be used to increase efficiency of a spatial domain.
An uplink control channel can be defined for transmission of an uplink control signal. Examples of the uplink control channel are various such as a fast feedback channel, a hybrid automatic repeat request (HARQ) feedback channel, a sounding channel, a ranging channel, a bandwidth request channel, etc. The fast feedback channel carries feedback of a channel quality indicator (CQI) and/or multiple-input multiple-output (MIMO) information, and can be classified into a primary fast feedback channel and a secondary fast feedback channel. The HARQ feedback channel is a channel for transmitting an acknowledgement (ACK)/non-acknowledgement (NACK) signal as a response for data transmission. The sounding channel can be used as an uplink channel response for closed-loop MIMO transmission and uplink scheduling. The bandwidth request channel is a channel for requesting a radio resource for transmitting a control signal or uplink data to be transmitted by a mobile station (MS).
The ranging channel can be used for uplink synchronization. The ranging channel can be classified into a ranging channel for a non-synchronized MS and a ranging channel for a synchronized MS. The ranging channel for the non-synchronized MS can be used for ranging on a target base station (BS) during initial network entry and handover. In a subframe in which the ranging channel for the non-synchronized MS is to be transmitted, the MS may not transmit any uplink burst or uplink control channel. The ranging channel for the synchronized MS can be used for periodic ranging. An MS which has already been synchronized with the target BS can transmit a ranging signal for the synchronized MS.
Meanwhile, various frame structures and allocated resources need to be taken into account when a base station (BS) allocates a ranging channel. When a ranging channel for a non-synchronized mobile station (MS) and a ranging channel for a synchronized MS are allocated together, the BS needs to allocate the channels by avoiding overlapping of the two channels.
Accordingly, there is a need for a method of allocating the ranging channel for the non-synchronized MS and the ranging channel for the synchronized MS by avoiding overlapping of resources to be allocated.